In many industrial chemical, thermal and physical processing systems, burners are used to produce gas streams having specific temperatures and compositions for a variety of applications involving momentum, heat and/or mass transfer processes. The costs of many of these material processing applications could be reduced if practical means for enhancing the rates of momentum, heat and mass transfer could be found. There is evidence in the prior art that the presence of pulsations in a gas flow results in large increases in the rates of momentum, heat and mass transfer processes.
Pulse combustors are known in the art as highly efficient sources of high temperature pulsating gas streams for heaters, boilers, and the like. Consequently, operational and capital investment costs of many industrial processes could be reduced if steady state burners commonly employed in such systems were replaced by pulse combustors which produce pulsating flows having the required thermal loads, temperature and compositions.
However, prior to the present invention pulse combustors were not optimally used in various industrial processes such as drying, calcining, heating and the like. Furthermore, prior to the present invention it has not been believed that pulse combustors could be designed to possess large turndown ratios, operate efficiently over wide ranges of fuel/air ratios and possess capabilities for controlling the amplitudes and frequencies of their pulsations. For example, in one prior art pulse dryer presently used for drying a slurry of kaolin, the slurry of material to be dried is injected directly into the tail pipe of a pulse combustor a short distance upstream of the pulse combustor exit plane. Upon leaving the pulse combustor, the pulsating flow and injected material enter a primary cyclone or drying chamber. The injection of material into the combustor tail pipe interferes with the combustor operation by adversely affecting its acoustic characteristics. This, in turn, limits the amount of material which can be dried and worsens the combustion process by decreasing the combustor capacity to ingest combustion air and achieve adequate mixing between the fuel and air. This results in incomplete combustion and undesirable soot formation in the combustor which adversely affects the properties of dried material, such as kaolin. Moreover, in this system the pulsations from the pulse combustor are damped out in the drying chamber, and no advantage whatsoever is taken of the natural acoustic characteristics of the drying chamber.
Other prior art material drying systems are known to use pulse combustors. In U.S. Pat. No. 3,618,655 to Lockwood, a paste of slurry of material to be dried is introduced into the exhaust pipe of a pulse jet engine, and the partly dried particles are then dispensed into a tank having vortices of gas at a substantially lower temperature than that found in the pulse jet exhaust. This structure is similar to the above-described kaolin drying system, and also appears unconcerned with the natural acoustic characteristics of the drying volume. In addition, there is a risk of overheating (with resultant burning of organic materials such as food products) in this type system, since the material is injected directly into the hot gas flow. Also, the system uses self aspirating pulse combustors which have limited ranges of operating conditions.
It is also known in the art to synchronize an oscillation-radiation chamber of a furnace with a pulsating combustion chamber. For example, in the papers of F. H. Reynst, there is described a system which employs a plurality of pulse combustors to excite a longitudinal acoustic mode inside a furnace chamber. By increasing or shortening the length of the oscillating column in the pulse combustors, the frequency is altered, thereby altering the oscillation induced in the radiation chamber. This system, however, appears limited to excitation of longitudinal acoustic modes in the furnace chamber. Moreover, the problems encountered in material processing environments, such as temperature, composition and moisture control and material drying time which are critical in, for example, dryers and calciners, are not considered in Reynst's writings.
It is also known in the art that transverse or "sloshing" type acoustic oscillations can be excited in cylindrical chambers and combustors. For example, the phenomenon of transverse oscillations was observed in studies of transverse instabilities in liquid fuel rocket motors.
It is also known in the art that undesirable acoustic oscillations, generally known as combustion instabilities, can be excited in ramjets, jet engine afterburners, solid propellant rocket motors and other propulsion systems by combustion processes inside shear layers which form downstream of flame holders, corners and mechanical obstacles. In studies directed towards the elimination of these undesirable combustion instabilities by, for example, Schadow and coworkers and Heitor et al., it has been shown that these instabilities are caused by interactions between vortex shedding at the origin of the shear layer, acoustic motions inside the propulsive device and unsteady combustion within vortices which are convected by the flow inside the shear layer. In all of these studies the investigated acoustic oscillations were restricted to the combustor under study and no teachings were provided as to how such pulsations can be used to improve industrial processes.
Prior to the present invention, however, there has been no attempt to utilize unsteady combustion processes inside shear layers, similar to those which excite undesirable combustion instabilities in propulsion systems, to excite flow pulsations within a processing chamber which will improve the rates of mass, momentum and heat transfer between the processed material and processing medium. Also, there has been no successful teaching that natural acoustic modes can be excited in a volume or system, such as a processing chamber, located downstream of the shear layer combustion region by an interaction between the acoustic mode oscillations excited inside the downstream volume and unsteady combustion within the shear layer. Also, prior to the present invention, there has been no teaching in the literature on how to construct pulse combustors which utilize unsteady combustion inside one or more shear layers to excite flow pulsations, and how to utilize such combustors to excite different acoustic mode oscillations in a processing chamber to which they are attached.